Systems and methods of optical path protection for distributed antenna systems

ABSTRACT

Systems and methods for optical path protection for distributed antenna systems are provided. In one embodiment, a method is provided. The method comprises receiving an electrical uplink radio frequency signal; generating an uplink optical signal derived from the electrical uplink radio frequency signal; splitting the uplink optical signal for transmission on a primary uplink optical fiber and a secondary uplink optical fiber; combining any downlink optical signal received on a primary downlink optical communication medium and any downlink optical signal received on a second downlink optical communication medium in order to output a downlink optical signal; and generating a downlink radio frequency signal derived from the downlink optical signal.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 11/380,798 filed on Apr. 28, 2006, entitled “SYSTEMS AND METHODS OF OPTICAL PATH PROTECTION FOR DISTRIBUTED ANTENNA SYSTEMS” (currently pending) which is hereby incorporated herein by reference.

BACKGROUND

Fiber breaks and/or degradation in a fiber feeding a distributed antenna system (DAS) network can have a large impact on a system's availability. Network operators typically look for a high-degree of network availability (for example, “0.9999” or higher). A network availability of 0.9999 requires a network be unavailable for no more than 1 hour per year, whereas a network availability of 0.99999 requires a network be unavailable for no more than 1 hour every ten years. One fiber break that causes a DAS network to be unavailable for 10 hours while the fiber break is isolated and repaired would lower availability to 0.999. Oftentimes fiber breaks take even longer to isolate and repair.

Adverse changes to a fiber-based network can happen in many different ways. Examples include digging near fiber optic cabling that causes damage to fiber, damaging the cable by a maintenance worker stepping on, bending, or breaking a fiber. Aerial-mounted fiber can be damaged during accidents involving the poles hanging the fiber. Optical connectors can become degraded by damage and/or dirt. These are just a few of the fiber degradation scenarios, any of which can cause degraded or loss of service on that fiber for extended periods of time. Due to the time required to isolate and repair fibers, redundant paths are often used in order to keep the system up as much as possible. Since fiber damage tends to be a local event, redundant fibers are usually run via completely different paths. In this way, any local damage done to the fiber does not affect the redundant path. In one application of a DAS network that makes use of redundant fibers, both a hub and remote node connected by the redundant fibers includes active switching devices in order to switch from a primary fiber to a secondary fiber in the event of a break (or other degradation of the primary fiber). However, remote nodes are often located in isolated areas with limited power resources and space for accommodating optical switching equipment.

SUMMARY

In one embodiment, a system is provided. The system comprises a hub; at least one remote node that is located remotely from the hub; wherein the hub communicates with the at least one remote node via either one of a primary fiber path and a backup fiber path, the primary fiber path comprising an uplink fiber and a downlink fiber and the backup fiber path comprising an uplink fiber and a downlink fiber; wherein the at least one remote node is coupled to the downlink fiber of the primary fiber path and the downlink fiber of the backup fiber path via an optical combiner; wherein the at least one remote node is further coupled to the uplink fiber of the primary fiber path and the uplink fiber of the backup fiber path via an optical splitter; and wherein the hub switches from communicating with the at least one remote node on the primary fiber path to communicating with the at least one remote node on the backup fiber path based on an uplink optical signal received from the at least one remote node.

In another embodiment, a method is provided. The method comprises receiving an electrical uplink radio frequency signal; generating an uplink optical signal derived from the electrical uplink radio frequency signal; splitting the uplink optical signal for transmission on a primary uplink optical fiber and a secondary uplink optical fiber; combining any downlink optical signal received on a primary downlink optical communication medium and any downlink optical signal received on a second downlink optical communication medium in order to output a downlink optical signal; and generating a downlink radio frequency signal derived from the downlink optical signal.

In another embodiment, an apparatus is provided. The apparatus comprises means for receiving an electrical uplink radio frequency signal; means for generating an uplink optical signal derived from the electrical uplink radio frequency signal; means for splitting the uplink optical signal for transmission on a primary uplink optical fiber and a secondary uplink optical fiber; means for combining any downlink optical signal received on the primary downlink optical communication medium and any downlink optical signal received on the second downlink optical communication medium in order to output a downlink optical signal; and means for generating a downlink radio frequency signal derived from the downlink optical signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of one embodiment of a distributed antenna system.

FIG. 2 is a block diagram of one embodiment of a distributed antenna system.

FIG. 3A is a block diagram of one embodiment of a distributed antenna system.

FIG. 3B is a block diagram of one embodiment of a distributed antenna system.

FIG. 4 is a flow chart of a one embodiment of a method of providing passive optical path protection.

FIG. 5 is a block diagram of one embodiment of a distributed antenna system.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of one embodiment of a distributed antenna system (DAS) 100 DAS 100 is often used in a wireless communication network (for example, a cellular wireless network) to communicatively couple one or more base stations (not shown in FIG. 1) to one or more antennas that are remotely located from the base stations (not shown in FIG. 1). DAS 100 includes a hub 110 located near the base stations and one or more remote access nodes 130 (also referred to here as “remote nodes” 130), each of which is located near one or more remote antennas (for example, a primary and diversity antenna). For the purposes of illustration, the DAS 100 comprises a single hub 110 that is optically coupled to single remote node 130 (though it is to be understood that in other embodiments other numbers of hubs 110 and/or remote nodes 130 are used). The hub 110 is optically coupled to the remote node 130 through a primary optical fiber path 120 and a backup optical fiber path 125. In the embodiment illustrated in FIG. 1, primary optical fiber path 120 includes a primary downlink optical fiber 122 and a primary uplink optical fiber 124. Backup optical fiber path 125 includes a secondary downlink optical fiber 127 and a secondary uplink optical fiber 129. In alternate embodiments, one or both of primary optical fiber path 120 and backup optical fiber path 125 include a plurality of uplink and/or downlink optical fibers.

Hub 110 comprises interface functionality 111 that couple the hub 110 to one or more base stations, a fiber optic transceiver 112 communicatively coupled to the one or more interface functionality 111, an optical switch 114 communicatively coupled to the fiber optic transceiver 112, primary optical fiber path 120 and backup optical fiber path 125, and a hub optical path protection (OPP) manager 116 communicatively coupled to the fiber optic transceiver 112 and the optical switch 114. In the downlink direction, the interface functionality 111 of the hub 110 receives analog downlink RF signals from the one or more base stations to which the hub 110 is communicatively coupled and provide the fiber optical transceiver 112 a suitable electrical signal for modulating onto a downlink optical carrier. The downlink optical signal output by the fiber optic transceiver 112 is selectively communicated to the remote node 130 on either the primary downlink optical fiber 122 or secondary downlink optical fiber 127 by the optical switch 114, depending on a fiber path control signal output by the hub optical path protection manager 116.

In one implementation of such an embodiment (also referred to here as a “digital-transport implementation”), the interface functionality 111 receives one or more analog downlink radio frequency (RF) signals from each of the base stations to which the hub 110 is communicatively coupled and digitizes at least a portion of the received analog downlink RF signal (for example, by digitizing a particular frequency band of the received analog downlink RF signal). Also, in such a digital-transport implementation, the interface functionality 111 combines at least a portion of the digitized downlink RF signals from one or more base stations into frames suitable for transmission on the primary downlink optical fiber 122 and/or the secondary downlink optical fiber 127 (for example, by formatting the at least a portion of the digitized downlink RF signals into SONET STS-48/OC-48 formatted frames). In such a digital-transport implementation, the fiber optic transceiver 112 digitally modulates the electrical signal (which comprises frames of digitized downlink RF signals) onto a downlink optical carrier in order to generate the downlink optical signal. An example of downlink functionality suitable for use in such a digital-transport implementation is described in U.S. Pat. No. 6,963,552, titled “MULTI-PROTOCOL DISTRIBUTED WIRELESS SYSTEM ARCHITECTURE” (also referred to here as the “'552 Patent”), which is hereby incorporated herein by reference.

In an alternative implementation of such an embodiment (also referred to here as an “analog-transport implementation”), the interface functionality 111 receives one or more analog downlink RF signals from each of the base stations to which the hub 110 is communicatively coupled and filters, combines, mixes, and/or splits the received analog downlink RF signals into a single electrical analog signal suitable for transmission on the primary downlink optical fiber 122 and/or the secondary downlink optical fiber 127. In such an analog-transport implementation, the fiber optic transceiver 112 amplitude modulates the single electrical analog signal received from the interface functionality 111 onto a downlink optical carrier in order to generate the downlink optical signal.

In the uplink direction, hub 110 receives an uplink optical signal from the remote node 130 on both of the primary uplink optical fiber 124 and the secondary uplink optical fiber 129. Optical switch 114 selectively couples one of the primary uplink optical fiber 124 and the secondary uplink optical fiber 129 to the fiber optic transceiver 112, depending on the fiber path control signal from the hub optical path protection manager 116. That is, when optical switch 114 selectively couples the primary uplink optical fiber 124 to the fiber optical transceiver 112, any uplink optical signal received on the primary uplink optical fiber 124 is communicated to the fiber optic transceiver 112 by the optical switch. Likewise, when optical switch 114 selectively couples the secondary uplink optical fiber 129 to the fiber optical transceiver 112, any uplink optical signal received on the secondary uplink optical fiber 129 is communicated to the fiber optic transceiver 112 by the optical switch. The fiber optic transceiver 112 demodulates the uplink optical signal in order to extract an electrical uplink RF signal, which is provided to one or more base stations via the interface functionality 111. In the digital-transport implementation of such an embodiment noted above, the extracted uplink RF signal comprises frames (for example, SONET STS-48/OC-48 formatted frames) containing digitized uplink RF signals, which the interface functionality 111 extracts from the frames and converts to analog uplink RF signals. The analog uplink RF signals, in such an embodiment, are provided to one or more base stations coupled to the hub 110. In the analog-transport implementation of such an embodiment noted above, the extracted uplink RF signal comprises an analog uplink RF signals that are provided to one or more base stations coupled to the hub 110.

Remote node 130 comprises an optical combiner 134, an optical splitter 135, a fiber optic transceiver 132, interface functionality 133, and a remote optical path protection (OPP) manager 136.

The optical combiner 134 of the remote node 130 receives downlink optical signals from hub 110 via one of either primary downlink optical fiber 122 or secondary downlink optical fiber 127. An optical “combiner”, as the term is used in this specification, means a device that receives a plurality of optical signal inputs and combines the optical signals into a single optical output. The downlink optical signal is transmitted on only one of the primary downlink optical fiber 122 or the secondary downlink optical fiber 127 at any one time. For this reason, at any one time, only the selected one of the primary downlink optical fiber 122 or the secondary downlink optical fiber 127 is “lit” while the fiber not selected by optical switch 114 is “dark” and does not carry an optical signal. This eliminates concerns of optical interference at the optical combiner 134. The resulting output of optical combiner 134, which is essentially the combination of the downlink optical signal from the lit fiber with a null signal from the dark fiber, is thus simply the downlink optical signal. The output of the optical combiner 134 (that is, the downlink optical signal) is communicated via a single fiber to fiber optic transceiver 132. The fiber optic transceiver 132 demodulates the downlink optical signal in order to extract an electrical downlink RF signal, which is provided to one or more antennas 140 and 141 via the interface functionality 133. In the digital-transport implementation of such an embodiment noted above, the electrical downlink RF signal output by the fiber optic transceiver 132 comprises digitized downlink RF signals from one or more base stations. In such an implementation, the interface functionality 133 converts the digitized downlink RF signals into analog downlink RF signals (for example, using a suitable digital-to-analog conversion process), which are amplified for radiation from the one or more antennas 140 and 141. In one example, the digitized downlink RF signals from each of the base stations is separately converted into an individual analog downlink RF signal for that respective base station (for example, by respective “RAN slices” of the type described in the '552 Patent) and each of the individual analog downlink RF signals are thereafter combined for amplification and radiation from one or more of the antennas 140 and 141. In the analog-transport implementation of such an embodiment noted above, the electrical downlink RF signal output by the fiber optic transceiver 132 comprises analog downlink RF signals from one or more of base stations and the interface functionality 133 amplifies for radiation from the one or more antennas 140 and 141.

In the uplink direction, the interface functionality 133 of the remote node 130 receives analog uplink RF signals from the one or more antennas 140 and 141 coupled to the remote node 130. The interface functionality 133 provides the fiber optical transceiver 132 a suitable electrical signal for modulating onto an uplink optical carrier. The uplink optical signal output by the fiber optic transceiver 132 is communicated on both of primary uplink optical fiber 124 and secondary uplink optical fiber 129 by the optical splitter 135. An optical “splitter”, as the term is used in this specification, means a device that replicates an optical signal received at an input to each of a plurality of outputs. That is, optical splitter 135 optically replicates the uplink optical signal output by the fiber optic transceiver 132 so that the uplink optical signal is communicated to hub 110 via both of the primary uplink optical fiber 124 and the secondary uplink optical fiber 129. In the digital-transport implementation of the embodiment shown in FIG. 1 noted above, the interface functionality 133 of the remote node 130 receives one or more analog uplink RF signals from each of the antennas 140 and 141 to which the remote node 130 is communicatively coupled and digitizes at least a portion of the received analog uplink RF signal (for example, by digitizing a particular frequency band of each analog uplink RF signal). Also, in such a digital-transport implementation, the interface functionality 133 combines at least a portion of the digitized uplink RF signals from one or more of the antennas 140 and 141 into frames suitable for transmission on the primary uplink optical fiber 124 and the secondary uplink optical fiber 129 (for example, by formatting the at least a portion of the digitized uplink RF signals into SONET STS-48/OC-48 formatted frames). In such a digital-transport implementation, the fiber optic transceiver 132 digitally modulates the electrical signal (which comprises frames of digitized uplink RF signals) onto an uplink optical carrier in order to generate the uplink optical signal.

Alternatively, in the analog-transport implementation of the embodiment shown in FIG. 1 noted above, the interface functionality 133 receives one or more analog uplink RF signals from each of the antennas 140 and 141 to which the remote node 130 is communicatively coupled and filters, combines, mixes, and/or splits the received analog uplink RF signals into a single analog signal suitable for transmission on the primary uplink optical fiber 124 and the secondary uplink optical fiber 129. In such an analog-transport implementation, the fiber optic transceiver 132 amplitude modulates the single electrical analog signal received from the interface functionality 133 onto an uplink optical carrier in order to generate the uplink optical signal, which is then communicated to the hub 110 on both the primary uplink optical fiber 124 and the secondary uplink optical fiber 129 via the splitter 135.

Under normal operating conditions, communications between hub 110 and remote node 130 are conducted over the primary optical fiber path 120. Normal operating conditions exist when communications between hub 110 and remote node 130 using the primary downlink optical fiber 122 and the primary uplink optical fiber 124 are accomplished within acceptable operating parameters discussed below.

The hub OPP manager (HOM) 116 determines when communications between hub 110 and remote node 130 via the primary fiber path 120 are within acceptable operating parameters. When HOM 116 determines that primary fiber path 120 is operating within acceptable operating parameters HOM 116 causes optical switch 114 to align itself for transmitting the downlink optical signal via the primary downlink optical fiber 122 and receiving the uplink optical signal from the primary uplink optical fiber 124 (the “normal state” of optical switch 114). When HOM 116 determines that communications via primary fiber path 120 are not within acceptable operating parameters, HOM 116 causes optical switch 114 to align itself for transmitting the downlink optical signal via the secondary downlink optical fiber 127 and to align itself for receiving the uplink optical signal from the secondary uplink optical fiber 129 (the “backup state” of optical switch 114).

HOM 116 determines when communications via primary fiber path 120 are not within acceptable operating parameters based on the uplink optical signal received on the primary uplink optical fiber 124.

In one implementation of DAS 100, fiber optic transceiver 112 communicates an uplink signal quality signal to HOM 116 (or other information indicative of an attribute of the uplink optical signal). When the uplink signal quality of the uplink optical signal degrades below a predetermined threshold level, HOM 116 switches optical switch 114 from its normal state to the backup state in order to utilize backup fiber path 125. In one implementation, fiber optic transceiver 112 communicates the bit error ratio (BER) of the uplink optical signal it receives. In such an implementation, when the BER drops below a predetermined threshold BER, HOM 116 switches optical switch 114 from its normal state to the backup state in order to utilize backup fiber path 125. As would be appreciated by one skilled in the art upon reading this specification, in a digital-transport implementation of a DAS 100, a typical good fiber path performs at better than a 10e-12 BER. Because of the amount of over-sampling typically used in such a digital-transport implementation, an optical fiber path can continue to operate down to as low as 10e-6 BER without adversely affecting call quality. In other implementations, other signal quality indicators, such as but not limited to optical power levels, are used to determine whether or not communications via primary fiber path 120 are within acceptable operating parameters. For example, in another implementation, fiber optic transceiver 112 measures and communicates the optical power level of the uplink optical signal it receives. In such an implementation, when the optical power level drops below a predetermined threshold power level, HOM 116 switches optical switch 114 from its normal state to the backup state in order to utilize backup fiber path 125.

Because hub 110 receives only the uplink optical signals, HOM 116 cannot directly determine the signal quality of downlink optical signals received by remote node 130. Therefore, information regarding the signal quality of downlink optical signals is communicated to HOM 116 by remote OPP manager (ROM) 136. In other embodiments, other information indicative of an attribute of the downlink optical signal is determined and communicated.

In one implementation, fiber optic transceiver 132 receives the downlink optical signal and communicates a downlink signal quality signal to ROM 136. In one implementation, fiber optic transceiver 132 communicates the bit error rate (BER) of the downlink optical signal it receives to ROM 136. In such an implementation, when the BER drops below a predetermined threshold level, ROM 136 reports the BER to HOM 116 so that HOM 116 can make the determination on whether to switch from the primary fiber path 120 to the backup fiber path 125. In other implementations, other signal quality indicators are used to determine whether or not communications via primary fiber path 120 are within acceptable operating parameters. When the downlink signal quality reported by ROM 136 degrades below a predetermined threshold level, HOM 116 switches optical switch 114 from its normal state to the backup state in order to utilize backup fiber path 125. For example, in another implementation, fiber optic transceiver 132 measures and communicates the optical power level of the downlink optical signal it receives. In such an implementation, when the optical power level drops below a predetermined threshold power level, ROM 136 reports the optical power level to HOM 116 so that HOM 116 can make the determination on whether to switch from the primary fiber path 120 to the backup fiber path 125.

As would be appreciated by one skilled in the art upon reading this specification, it is not necessary for uplink and downlink signal quality to be based on the same criteria. For example, in one implementation fiber optic transceiver 112 measures and communicates the optical power level of the uplink optical signal it receives while fiber optic transceiver 132 communicates the bit error rate (BER) of the downlink optical signal it receives. In another implementation fiber optic transceiver 112 communicates the bit error rate (BER) of the uplink optical signal it receives while fiber optic transceiver 132 communicates the optical power level of the downlink optical signal it receives.

In another implementation, ROM 136 makes the determination whether to switch communications from the primary fiber path 120 to the backup fiber path 125 based on the signal quality of the downlink optical signal, and alerts HOM 116 of a degraded downlink fiber path by shutting off the uplink fiber path. In one such implementation, when the BER of the downlink optical signal drops below the predetermined threshold level, ROM 136 disables fiber optic transceiver 132 from transmitting the uplink optical signal to hub 110. In another such implementation, when the optical power level of the downlink optical signal drops below the predetermined threshold level, ROM 136 disables fiber optic transceiver 132 from transmitting the uplink optical signal to hub 110. Fiber optic transceiver 112 detects this loss of the uplink optical signal and outputs an uplink signal quality signal that indicates the loss of the uplink optical signal. HOM 116 responds to what it perceives as a degraded uplink signal quality by switching optical switch 114 from its normal state to the backup state in order to utilize backup fiber path 125.

In the embodiment shown in FIG. 1, all the fiber switching is performed at the hub 110 rather than at remote node 130. The hub 110 contains all of the “active” optical path protection circuitry (that is, the optical switch 114) that is used to switch among the redundant fiber paths. The remote node 130 includes passive optical splitters and combiners that typically do not require significant space to install (space often being a scarce resource at remote node locations) and typically do not require addition power to operate (power often being a scarce resource at remote node locations). Further, when space is very limited within a remote node, the optical splitter and optical combiner need not be located within the remote node housing, but can be installed external to the remote node housing, as illustrated in the embodiment shown in FIG. 2. FIG. 2 illustrates a hub 210 coupled to an optical combiner 234 and an optical splitter 235 via a primary optical fiber path 220 and a backup fiber path 225. Hub 210 operates as described with respect to hub 110 shown in FIG. 1, transmitting downlink optical signals to optical combiner 234 and receiving uplink optical signals from optical splitter 235. As illustrated in FIG. 2, optical combiner 234 and optical splitter 235 are located in a housing 215 that is external to remote node 230. Optical combiner 234 and optical splitter 235 are coupled to the fiber optic transceiver 232 within remote node 230 via fiber pair 220. Because optical combiner 234 and optical splitter 235 do not require power to operate, housing 215 may comprise an underground cable vault or similar location where power is not available. Further, only a single fiber pair 220 needs to be run between housing 215 and the remote node 230, thus reducing system installation costs. A ROM 236, interface functionality 233, and fiber optic transceiver 232 within remote node 230 otherwise operate as described with respect to ROM 136, interface functionality 133, and fiber optic transceiver 132 in FIG. 1. The approach shown in FIG. 2 is also suitable for use in “retro-fitting” or “aftermarket” applications where a service provider desires to add optical path protection for deployed remote nodes that are not designed to house an optical combiner or an optical splitter. In such an application, the functionality of the ROM is implemented in an element management system that is otherwise coupled to the remote nodes, and the protection switching performed by the optical switch in the respective hub is controlled by communications from the element management system that is otherwise communicatively coupled to the hub.

Although the embodiment shown in FIG. 1 is described, for the purposes of illustration, as using a single optical wavelength on each fiber, it is to be understood that in other embodiments multiple optical wavelengths can be used. One such embodiment is illustrated in FIG. 3A. In the embodiment of FIG. 3A, hub 310 communicates with a plurality of remote nodes 330-1 to 330-N via multiple wavelength optical signals. Hub 310 includes the functionality required to multiplex multiple optical carrier signals from a communications network (not shown) onto a single optical fiber by using different wavelengths of light to carry a plurality of different RF signals. Hub 310 comprises interface functionality 311-1 to 311-x that couple the hub 310 to one or more base stations (not shown). The hub 310 further comprises fiber optic transceivers 312-1 to 312-x communicatively coupled, respectively, to the interface functionality 311-1 to 311-x, and wave division multiplexing (WDM) multiplexer (MUX) 313 communicatively coupled to the downlink output of fiber optical transceivers 312-1 to 312-x. The hub 310 further comprises a WDM DE-MUX 315 communicatively coupled to the uplink inputs of optical transceivers 312-1 to 312-x and an optical switch 314 communicatively coupled to WDM MUX 313, WDM DE-MUX 315, primary optical fiber path 320 and backup optical fiber path 325. The hub 310 further comprises a hub optical path protection manager (HOM) 316 communicatively coupled to fiber optic transceivers 312-1 to 312-x and optical switch 314.

In the downlink direction, the interface functionality 311-1 to 311-x receives analog downlink RF signals from the one or more base stations to which the hub 310 is communicatively coupled and provides to the fiber optical transceivers 312-1 to 312-x, respectively, a suitable electrical signal for modulating onto a downlink optical carrier. The fiber optical transceivers 312-1 to 312-x modulate the electrical signal onto a different downlink optical carrier. The downlink optical signal d1, d2, to dx output by each of the fiber optic transceivers 312-1 to 312-x is optically multiplexed by WDM MUX 313 into a single downlink multiple wavelength optical signal (shown as d1+d2+. . . +dx). The downlink multiple wavelength optical signal output by WDM MUX 313 is selectively communicated to the remote node 330 on either the primary downlink optical fiber 322 or secondary downlink optical fiber 327 by the optical switch 314, depending on a fiber path control signal output by the hub optical path protection manager 316.

In one digital-transport implementation of such an embodiment, the interface functionality 311-1 to 311-x receives one or more analog downlink radio frequency (RF) signals from each of the base stations to which the hub 310 is communicatively coupled and digitizes at least a portion of the received analog downlink RF signals (for example, by digitizing a particular frequency band of each received analog downlink RF signal). Also, in such a digital-transport implementation, each item of interface functionality 311-1 to 311-x combines at least a portion of the digitized downlink RF signals from one or more base stations into frames suitable for transmission on the primary downlink optical fiber 322 or the secondary downlink optical fiber 327 (for example, by formatting the at least a portion of the digitized downlink RF signals into SONET STS-48/OC-48 formatted frames). In such a digital-transport implementation, the fiber optic transceivers 312-1 to 312-x each digitally modulate the electrical signal (which comprises frames of digitized downlink RF signals) onto a respective downlink optical carrier. All of the downlink optical signals output by the fiber optic transceivers 312-1 to 312-x are multiplexed together by the WDM MUX 313. An example of downlink functionality suitable for use in such a digital-transport implementation is described in U.S. Pat. No. 6,963,552, titled “MULTI-PROTOCOL DISTRIBUTED WIRELESS SYSTEM ARCHITECTURE” (also referred to here as the “'552 Patent”), which is hereby incorporated herein by reference.

In an alternative analog-transport implementation of such an embodiment, the interface functionality 311-1 to 311-x each receives one or more analog downlink RF signals from each of the base stations to which the hub 310 is communicatively coupled and filters, combines, mixes, and/or splits the received analog downlink RF signals into a single electrical analog signal suitable for transmission on the primary downlink optical fiber 322 and/or the secondary downlink optical fiber 327. In such an analog-transport implementation, the fiber optic transceivers 312-1 to 312-x each amplitude modulate the single electrical analog signal received from the respective items of interface functionality 311-1 to 311-x onto a respective downlink optical carrier in order to generate the respective downlink optical signal. All of the downlink optical signals output by the fiber optic transceivers 312-1 to 312-x are multiplexed together by the WDM MUX 313. In the embodiment shown in FIG. 3A, the downlink output of hub 310 is coupled to the remote nodes 330-1 to 330-N via a cable vault 331 that is remotely located from the hub 310 and proximal to the first remote node 330-1. The cable vault 331 comprises an optical combiner 334 that has two inputs that are coupled to the primary downlink optical fiber 322 and the secondary downlink optical fiber 327. Optical switch 314 outputs the downlink multiple wavelength optical signal on either the primary downlink optical fiber 322 or the secondary downlink optical fiber 327, depending on the fiber path control signal from the hub optical path protection manager 316.

In the uplink direction, hub 310 receives an uplink multiple wavelength optical signal via one or both of primary uplink optical fiber 324 and secondary uplink optical fiber 329 from an optical splitter 335 that is also housed within the cable vault 331. The uplink multiple wavelength optical signal comprises multiplexed uplink optical signals (shown as u1, u2, . . . ux) from the remote nodes 330-1 to 330-N. Optical switch 314 selectively couples one of the primary uplink optical fiber 324 and the secondary uplink optical fiber 329 to the WDM DE-MUX 315, depending on the fiber path control signal from the hub optical path protection manager 316. That is, when optical switch 314 selectively couples the primary uplink optical fiber 324 to the WDM DE-MUX 315, any uplink multiple wavelength optical signal received on the primary uplink optical fiber 324 is communicated to the WDM DE-MUX 315 by the optical switch 314. Likewise, when optical switch 314 selectively couples the secondary uplink optical fiber 329 to the WDM DE-MUX 315, any uplink multiple wavelength optical signal received on the secondary uplink optical fiber 329 is communicated to the WDM DE-MUX 315 by the optical switch 314. WDM DE-MUX 315 de-multiplexes the uplink multiple wavelength optical signal into x single wavelength optical signals, and forwards each single wavelength optical signal to one of the fiber optic transceivers 312-1 to 312-x. Each of the fiber optic transceivers 312-1 to 312-x demodulates the uplink optical signal they receive from WDM DE-MUX 315 in order to extract an electrical uplink RF signal, which is respectively provided to one or more base stations via the interface functionality 311-1 to 311-x. In the digital-transport implementation of such an embodiment noted above, the extracted uplink RF signal comprises frames (for example, SONET STS-48/OC-48 formatted frames) containing digitized uplink RF data, which the interface functionality 311-1 to 311-x extracts from the frames and converts to analog uplink RF signals. The analog uplink RF signals, in such an implementation, are provided to one or more base stations coupled to the hub 310. In the analog-transport implementation of such an embodiment noted above, the extracted uplink RF signal comprises analog uplink RF signals that are provided to one or more base stations coupled to the hub 310 (for example, with appropriate amplification and filtering). In the embodiment shown in FIG. 3A, optical combiner 334 operates as described above with respect to optical combiners 134 and 235 to combine any downlink multiple wavelength optical signal received on primary downlink optical fiber 322 and secondary downlink optical fiber 327 into a single downlink multiple wavelength optical output. Optical splitter 335 operates as described above with respect to optical splitters 135 and 235 to replicate any uplink multiple wavelength optical signal for communication to hub 310 via both of the primary uplink optical fiber 324 and the secondary uplink optical fiber 329.

In the embodiment shown in FIG. 3A, remote nodes 330-1 to 330-N are communicatively coupled to one another in a “daisy chain” topology. Each of the remote nodes 330-1 to 330-N is coupled to optical combiner 334 and optical splitter 335 through add/drop multiplexers 350-1 to 350-M.

In the downlink direction, a first add/drop multiplexer 350-1 is coupled to optical combiner 334 and receives the downlink multiple wavelength signal that comprises a multiplexed version of the RF data signal outputs from fiber optic transceivers 312-1 to 312-x (shown as d1+d2+ . . . +dx). Add/drop multiplexer 350-1 “drops” downlink optical signal d1 (which is the downlink optical signal output by the fiber optic transceiver 330-1) to first remote node 330-1 over a downlink optical fiber. In the implementation shown in FIG. 3A, Add/drop multiplexer 350-1 also outputs the received downlink multiple wavelength signal minus the downlink optical signal dl dropped to the first remote node 330-1 (shown as d2+ . . . +dx)to the next add/drop multiplexer in the daisy chain. The next add/drop multiplexer in the downlink direction receives the signal from the first add/drop multiplexer, similarly drops the second downlink optical signal d2 to the second remote node 330-2, and outputs the remaining signal to next add/drop multiplexer in the daisy chain. Each of the add/drop multiplexers in the daisy chain similarly drop the respective downlink optical signal to the respective remote unit until the last remote node 330-N receives the last downlink optical signal (shown as dx). In alternate implementations, the downlink optical signals are not necessarily removed from the fiber as they are dropped to their associated remote nodes. For example, in one such alternate implementation, Add/drop multiplexer 350-1 outputs the same downlink multiple wavelength signal it received to the next add/drop multiplexer in the daisy chain.

In the uplink direction each of the add/drop multiplexers 350-1 to 350-M adds a respective uplink optical signal received from a respective remote node to the previously multiplexed optical signals from the daisy chain. For example, add/drop multiplexer 350-M multiplexes together uplink optical signals received from the last remote node 330-N and the second to last remote node 330-(N−1) to produce an uplink multiple wavelength optical signal (illustrated in FIG. 3A as u(x−1)+ux). The next upstream add/drop multiplexer receives that uplink multiple wavelength optical signal u(x−1)+ux and adds it together with an uplink optical signal received from its associated remote node. Thus the uplink multiple wavelength optical signal received by optical splitter 335 comprises a multiplexed version of the uplink multiple wavelength optical signal (shown as u1+u2+ . . . +ux).

Each of the remote nodes 330-1 to 330-N comprise a fiber optic transceiver, interface functionality, and a remote optical path protection (OPP) manager that function as described with respect to remote node 230 illustrated in FIG. 2. For example, remote node 330-1, evaluates the downlink signal quality of the downlink optical signal received from the hub 310. In one implementation, the remote nodes 330-1-330-N determines the bit error rate (BER) of the respective downlink optical signal received at that remote node. When the BER measured by from one or more of the remote nodes 330-1-330-N drops below a predetermined threshold level, that remote node reports the BER to hub 310 by any of the means described with respect to FIGS. 1 and 2. Hub 310 can then make the determination on whether to realign optical switch 314 from the primary fiber path 320 to the backup fiber path 325. In other implementations, other signal quality indicators are used to determine whether or not communications via primary fiber path 320 are within acceptable operating parameters. For example, in another implementation each of the remote nodes 330-1-330-N determine the optical power level of the downlink optical signal it receives. In such an implementation, when the optical power level drops below a predetermined threshold power level, the remote nodes reports the optical power level to hub 310 by any of the means described with respect to FIGS. 1 and 2. Hub 310 can then make the determination on whether to realign optical switch 314 from the primary fiber path 320 to the backup fiber path 325.

In one implementation, hub 310 determines whether to switch from primary fiber path 320 to the backup fiber path 325 based on all of the signal quality feedback provided for the remote nodes 330-1 to 330-N. For example, in one implementation, if a downlink BER or optical power level reported by any one of the remote nodes 330-1 to 330-N indicates degrading optical signal quality, but downlink BERs or optical power levels reported by the other remote nodes do not, the hub 310 concludes that the degrading optical signal quality is due to a local problem with the one remote node, rather than a degradation of the primary fiber path 320. However, when all of the remote nodes 330-1 to 330-N report degrading downlink optical signal quality, then hub 310 concludes that the degrading optical signal quality is due to a degradation of the primary fiber path 320 and switches to the backup fiber path 325. Similarly, if hub 310 detects the loss of an uplink optical signal from one of the remote nodes 330-1 to 330-N, but continues to detect uplink optical signals from the other remote nodes, the hub 310 concludes that the degrading optical signal quality is due to a local problem with that remote node, rather than a degradation of the primary fiber path 320. However, when hub 310 detects a loss of uplink optical signals from all of the remote nodes 330-1 to 330-N, then hub 310 concludes that the degrading optical signal quality is due to a degradation of the primary fiber path 320 and switches to the backup fiber path 325.

Although the embodiments illustrated in this specification describe optical path protection systems and methods in terms of switching from a designated primary fiber path to a designated backup fiber path upon detection of primary fiber path degradation, one of ordinary skill in the art upon reading this specification would appreciate that the same systems and methods are applicable for switching from the designated backup fiber path back to the designated primary fiber path. Further, the designation in the field of which fiber path is the primary fiber path and which is the backup fiber path is arbitrarily determined by the system operator. As such embodiments of the present invention include embodiments for switching between any first fiber path and any second fiber path. In some embodiments, hysteresis is including in the switching determination made by a hub to prevent continual switching (also called ‘Flapping’) between two fiber paths that are both degraded but not failed.

FIG. 3A illustrates and embodiment where each of the remote units 330-1 to 330-N is associated with its own add/drop multiplexer. FIG. 3B illustrates one embodiment where a single mux/demux 355 is located at a convenient installation near remote units 330-1 to 330-N. Instead of daisy chaining the multiple add/drop multiplexers 350-1 to 350-M, to communicate the uplink optical signals (u1 to ux) and downlink optical signals (d1 to dx) with remote units 330-1 to 330-N, in the embodiment of FIG. 3B, mux/demux 355 is directly coupled to each of remote units 330-1 to 330-N, optical combiner 334 and optical splitter 335. Mux/demux 355 includes the functionality required to receive the downlink multiple wavelength signal (shown as d1+d2+ . . . +dx)from optical combiner 334 and distribute each of the downlink optical signals (d1 to dx) within the downlink multiple wavelength signal to their corresponding remote unit of remote units 330-1 to 330-N. Mux/demux 355 includes the functionality required to receive the uplink optical signals (u1 to ux) from remote units 330-1 to 330-N, multiplex the uplink optical signals to into the uplink multiple wavelength signal (shown as u1+u2+ . . . +ux), and communicate the uplink multiple wavelength signal to optical splitter 335.

FIG. 4 is a flow chart illustrating a method for providing optical path protection for a distributed antenna system. In one implementation, the method is performed by a distributed antenna system as described with respect to FIGS. 1, 2 and 3. The method begins at 410 with transmitting a downlink optical signal to a downlink fiber of one of a first fiber path and a second fiber path. In one implementation, the first fiber path functions as a primary fiber path while the second fiber path functions as a backup fiber path. In other implementations, the first fiber path functions as backup fiber path while the second fiber path functions as the primary fiber path. In one implementation, transmitting the downlink optical signal is performed from a hub of the distributed antenna system. The method continues at 415 with combining any downlink optical signal received on the downlink fiber of the first fiber path and any downlink optical signal received on the downlink fiber of the second fiber path in order to output the downlink optical signal. Combining any downlink optical signals from the first and second fiber paths at 415 while transmitting a downlink optical signal at 410 to only one of those paths enables the passive reception of the downlink optical signal without a-priori knowledge of whether the downlink optical signal is transmitted on the first or second fiber path. The method continues at 420 with splitting an uplink optical signal for transmission on an uplink fiber of the first fiber path and an uplink fiber of the second fiber path. Splitting the uplink optical signal for transmission on both the first and second optical fiber paths enables the passive transmission of the uplink optical signal from a remote node without the need for a-priori knowledge of whether a hub is aligned to receive uplink optical signals from the first or second optical fiber path. In one implementation, transmitting the uplink optical signal is performed from a remote node of the distributed antenna system. The method proceeds to 430 with determining a signal quality of the downlink optical signal. In one implementation of the method, determining the signal quality of the downlink optical signal comprises calculating the bit error rate of the downlink optical signal. In one implementation of the method, determining the signal quality of the downlink optical signal comprises determining the optical power level of the downlink optical signal. In one implementation, when the downlink optical signal comprises multiple wavelengths, determining the signal quality of the downlink optical signal comprises determining the bit error rate for data communicated in one of the multiple wavelengths. In one implementation, a remote node communicates the signal quality of the downlink optical signal back to the hub. Communication of the signal quality to the hub can include transmitting signal quality data to the hub via an uplink optical signal, or alerting the hub of inadequate downlink optical signal quality by disabling the uplink optical signal. The method proceeds to 440 with determining a signal quality of the uplink optical signal. In one implementation of the method, determining the signal quality of the uplink optical signal comprises calculating the bit error rate of the uplink optical signal. In one implementation, when the uplink optical signal comprises multiple wavelengths, determining the signal quality of the downlink optical signal comprises determining the bit error rate for data communicated in one of the multiple wavelengths. The method proceeds to 450 with switching between a normal state and backup state based on one or both of the signal quality of the uplink optical signal and the signal quality of the downlink optical signal. In one implementation of the method when switched to a normal state, downlink optical signals are transmitted on the downlink fiber of the first fiber path and uplink optical signals are received on the uplink fiber of the first fiber path. In such an implementation, when switched to the backup state, downlink optical signals are transmitted on the downlink fiber of the second fiber path and uplink optical signals are received on the uplink fiber of the second fiber path. In another implementation of the method, when switched to the normal state, downlink optical signals are transmitted on the downlink fiber of the second fiber path and uplink optical signals are received on the uplink fiber of the second fiber path. In such an implementation of the method, when switched to the backup state, downlink optical signals are transmitted on the downlink fiber of the first fiber path and uplink optical signals are received on the uplink fiber of the first fiber path.

In one implementation, when a hub is switched to utilize the first fiber path and either the signal quality of the downlink optical signal (determined at 430) or the signal quality of the uplink optical signal (determined at 440) is inadequate, the hub switches from the first fiber path to the second fiber bath. Conversely, when the hub is switched to utilize the second fiber path and either the signal quality of the downlink optical signal (determined at 430) or the signal quality of the uplink optical signal (determined at 440) is inadequate, the hub switches from the second fiber path to the first fiber path. In one implementation of the method, hysteresis is including in the switching determination made by a hub to prevent continual switching between the first and second fiber paths when both are degraded but not failed.

As would be appreciated by one skilled in the art upon reading this specification, a single fiber can be used to communicate both an uplink optical signal and a downlink optical signal simultaneously by appropriate allocation of wavelengths. Such implementations are included within the scope of embodiments of the present invention, as illustrated by the embodiment of FIG. 5. In the embodiment of FIG. 5, hub 510 communicates with a plurality of remote nodes 530-1 to 530-N via multiple wavelength optical signals. Hub 510 includes the functionality required to multiplex multiple optical carrier signals from a communications network (not shown) onto a single optical fiber by using different wavelengths of light to carry a plurality of different RF signals. Hub 510 comprises interface functionality 511-1 to 511-x that couple the hub 310 to one or more base stations (not shown). The hub 510 further comprises fiber optic transceivers 512-1 to 512-x communicatively coupled, respectively, to the interface functionality 511-1 to 511-x. The hub 510 further comprises a hub optical path protection manager (HOM) 516 communicatively coupled to fiber optic transceivers 512-1 to 512-x and optical switch 514.

Hub 510 further comprises a wave division multiplexing (WDM) multiplexer/demultiplexer (MUX/DE-MUX) 513 that is communicatively coupled to the downlink output of fiber optical transceivers 312-1 to 312-x and the uplink inputs of optical transceivers 312-1 to 312-x, and further comprises an optical switch 514 communicatively coupled to WDM MUX/DE-MUX 513, primary optical fiber path 520 and backup optical fiber path 525. In the embodiment of FIG. 5, primary optical fiber path 520 comprises a common fiber used to communicate both an uplink multiple wavelength signal (shown as u1+u2+ . . . +ux) from hub 510 to remote nodes 530-1 to 530-N and a downlink multiple wavelength signal (shown as d1+d2+ . . . +dx) from the remote nodes 530-1 to 530-N to hub 510. Backup optical fiber path 525 comprises another common fiber used to communicate both an uplink multiple wavelength signal from hub 510 to remote nodes 530-1 to 530-N and a downlink multiple wavelength signal from the remote nodes 530-1 to 530-N to hub 510.

In the downlink direction, the interface functionality 511-1 to 511-x receives analog downlink RF signals from the one or more base stations to which the hub 510 is communicatively coupled and provides to the fiber optical transceivers 512-1 to 512-x, respectively, a suitable electrical signal for modulating onto a downlink optical carrier. The fiber optical transceivers 512-1 to 512-x modulate the electrical signal onto a different downlink optical carrier. The downlink optical signal (d1, d2, to dx) output by each of the fiber optic transceivers 512-1 to 512-x is optically multiplexed by WDM MUX/DE-MUX 513 into the single downlink multiple wavelength optical signal (shown as d1+d2+ . . . +dx). The downlink multiple wavelength optical signal output by WDM MUX/DE-MUX 513 is selectively communicated to the remote node 330 on either the primary optical fiber path 520 or the backup optical fiber path 525 by the optical switch 514, depending on a fiber path control signal output by the hub optical path protection manager 516.

In one digital-transport implementation of such an embodiment , the interface functionality 511-1 to 511-x receives one or more analog downlink radio frequency (RF) signals from each of the base stations to which the hub 510 is communicatively coupled and digitizes at least a portion of the received analog downlink RF signals (for example, by digitizing a particular frequency band of each received analog downlink RF signal). Also, in such a digital-transport implementation, each item of interface functionality 511-1 to 511-x combines at least a portion of the digitized downlink RF signals from one or more base stations into frames suitable for transmission on the primary optical fiber path 520 or the backup optical fiber path 525 (for example, by formatting the at least a portion of the digitized downlink RF signals into SONET STS-48/OC-48 formatted frames). In such a digital-transport implementation, the fiber optic transceivers 512-1 to 512-x each digitally modulate the electrical signal (which comprises frames of digitized downlink RF signals) onto a respective downlink optical carrier. All of the downlink optical signals output by the fiber optic transceivers 512-1 to 512-x are multiplexed together by the WDM MUX/DE-MUX 513. An example of downlink functionality suitable for use in such a digital-transport implementation is described in U.S. Pat. No. 6,963,552, titled “MULTI-PROTOCOL DISTRIBUTED WIRELESS SYSTEM ARCHITECTURE” (also referred to here as the “'552 Patent”), which is hereby incorporated herein by reference.

In an alternative analog-transport implementation of such an embodiment, the interface functionality 511-1 to 511-x each receives one or more analog downlink RF signals from each of the base stations to which the hub 510 is communicatively coupled and filters, combines, mixes, and/or splits the received analog downlink RF signals into a single electrical analog signal suitable for transmission on the primary optical fiber path 520 or the backup optical fiber path 525. In such an analog-transport implementation, the fiber optic transceivers 512-1 to 512-x each amplitude modulate the single electrical analog signal received from the respective items of interface functionality 511-1 to 511-x onto a respective downlink optical carrier in order to generate the respective downlink optical signal. All of the downlink optical signals output by the fiber optic transceivers 512-1 to 512-x are multiplexed together by the WDM MUX/DE-MUX 513. In the embodiment shown in FIG. 5, the downlink output of hub 510 is coupled to the remote nodes 530-1 to 530-N via a cable vault 531 that is remotely located from the hub 510 and proximal to the first remote node 530-1. The cable vault 531 comprises an optical combiner/splitter 534 that has two inputs that are coupled to the primary optical fiber path 520 and the backup optical fiber path 525. Optical switch 514 outputs the downlink multiple wavelength optical signal on either the primary optical fiber path 520 or the backup optical fiber path 525, depending on the fiber path control signal from the hub optical path protection manager 516.

In the uplink direction, hub 510 receives an uplink multiple wavelength optical signal via one or both of primary optical fiber path 520 and the backup optical fiber path 525 from optical combiner/splitter 534. The uplink multiple wavelength optical signal comprises multiplexed uplink optical signals (shown as u1, u2, . . . ux) from the remote nodes 530-1 to 530-N. Optical switch 514 selectively couples one of the primary optical fiber path 520 or the backup optical fiber path 525 to the WDM MUX/DE-MUX 513, depending on the fiber path control signal from the hub optical path protection manager 516. That is, when optical switch 514 selectively couples the primary optical fiber path 520 to the WDM MUX/DE-MUX 513, any uplink multiple wavelength optical signal received on the primary optical path 520 is communicated to the WDM MUX/DE-MUX 513 by the optical switch 514. Likewise, when optical switch 514 selectively couples the backup optical path 525 to the WDM MUX/DE-MUX 513, any uplink multiple wavelength optical signal received on the backup optical path 525 is communicated to the WDM MUX/DE-MUX 513 by the optical switch 514.

WDM MUX/DE-MUX 513 de-multiplexes the uplink multiple wavelength optical signal into x single wavelength optical signals, and forwards each single wavelength optical signal to one of the fiber optic transceivers 512-1 to 512-x. Each of the fiber optic transceivers 512-1 to 512-x demodulates the uplink optical signal they receive from WDM MUX/DE-MUX 315 in order to extract an electrical uplink RF signal, which is respectively provided to one or more base stations via the interface functionality 511-1 to 511-x. In the digital-transport implementation of such an embodiment noted above, the extracted uplink RF signal comprises frames (for example, SONET STS-48/OC-48 formatted frames) containing digitized uplink RF data, which the interface functionality 511-1 to 511-x extracts from the frames and converts to analog uplink RF signals. The analog uplink RF signals, in such an implementation, are provided to one or more base stations coupled to the hub 510. In the analog-transport implementation of such an embodiment noted above, the extracted uplink RF signal comprises analog uplink RF signals that are provided to one or more base stations coupled to the hub 510 (for example, with appropriate amplification and filtering). In the embodiment shown in FIG. 5, optical combiner/splitter 534 operates in the downlink direction as described above with respect to optical combiners 134 and 235 to combine any downlink multiple wavelength optical signal received on primary optical path 520 and backup optical path 525 into a single downlink multiple wavelength optical output. Optical combiner/splitter 535 operates in the uplink direction as described above with respect to optical splitters 135 and 235 to replicate any uplink multiple wavelength optical signal for communication to hub 510 via both of the primary optical path 520 and the backup optical path 525.

In the embodiment shown in FIG. 5, remote nodes 530-1 to 530-N are communicatively coupled to one another in a “daisy chain” topology. Each of the remote nodes 530-1 to 530-N is coupled to optical combiner/splitter 534 through add/drop multiplexers 550-1 to 550-M. In the embodiment shown in FIG. 5, each of the add/drop multiplexers 550-1 to 550-M are coupled to their adjacent add/drop multiplexer using a common used to communicate both the uplink optical signals and downlink optical signals simultaneously by appropriate allocation of wavelengths. The first add/drop multiplexer 550-1 is coupled to optical combiner/splitter 534 via a common fiber used to receive the downlink multiple wavelength signal from the optical combiner/splitter 534 and to send the uplink downlink multiple wavelength signal to the optical combiner/splitter 534.

Add/drop multiplexers 550-1 to 550-M each include the functionality described with respect to add/drop multiplexers 350-1 to 350-M. In the downlink direction first add/drop multiplexer 550-1 receives the downlink multiple wavelength signal (shown as d1+d2+ . . . +dx), and “drops” downlink optical signal dl to first remote node 530-1 over an optical fiber. In the implementation shown in FIG. 5, add/drop multiplexer 550-1 also outputs the received downlink multiple wavelength signal minus the downlink optical signal dl dropped to the first remote node 530-1 (shown as d2+ . . . +dx) to the next add/drop multiplexer in the daisy chain. The next add/drop multiplexer in the downlink direction receives the signal from the first add/drop multiplexer, similarly drops the second downlink optical signal d2 to the second remote node 530-2, and outputs the remaining signal to next add/drop multiplexer in the daisy chain. Each of the add/drop multiplexers in the daisy chain similarly drop the respective downlink optical signal to the respective remote unit until the last remote node 530-N receives the last downlink optical signal (shown as dx). In alternate implementations, the downlink optical signals are not necessarily removed from the fiber as they are dropped to their associated remote nodes. For example, in one such alternate implementation, Add/drop multiplexer 550-1 outputs the same downlink multiple wavelength signal it received to the next add/drop multiplexer in the daisy chain.

In the uplink direction each of the add/drop multiplexers 550-1 to 550-M adds a respective uplink optical signal received from a respective remote node to the previously multiplexed optical signals from the daisy chain. In one implementation an add/drop multiplexer receives an uplink optical signal from their associated remote node over the same optical fiber used to send the downlink optical signal to the remote node. For example, add/drop multiplexer 550-M multiplexes together uplink optical signals received from the last remote node 530-N and the second to last remote node 530-(N−1) to produce an uplink multiple wavelength optical signal (illustrated in FIG. 5 as u(x−1)+ux). The next upstream add/drop multiplexer receives that uplink multiple wavelength optical signal u(x−1)+ux and adds it together with an uplink optical signal received from its associated remote node. Thus the uplink multiple wavelength optical signal received by optical splitter 535 comprises a multiplexed version of the uplink multiple wavelength optical signal (shown as u1+u2+ . . . +ux).

Each of the remote nodes 530-1 to 530-N comprise a fiber optic transceiver, interface functionality, and a remote optical path protection (OPP) manager that function as described with respect to remote node 230 illustrated in FIG. 2. For example, remote node 530-1, evaluates the downlink signal quality of the downlink optical signal received from the hub 510. In one implementation, the remote nodes 530-1-530-N determines the bit error rate (BER) of the respective downlink optical signal received at that remote node. When the BER measured by from one or more of the remote nodes 530-1-530-N drops below a predetermined threshold level, that remote node reports the BER to hub 510 by any of the means described with respect to FIGS. 1 and 2. Hub 510 can then make the determination on whether to realign optical switch 514 from the primary fiber path 520 to the backup fiber path 525. In other implementations, other signal quality indicators are used to determine whether or not communications via primary fiber path 520 are within acceptable operating parameters. For example, in another implementation each of the remote nodes 530-1-530-N determine the optical power level of the downlink optical signal it receives. In such an implementation, when the optical power level drops below a predetermined threshold power level, the remote nodes reports the optical power level to hub 510 by any of the means described with respect to FIGS. 1 and 2. Hub 510 can then make the determination on whether to realign optical switch 514 from the primary fiber path 520 to the backup fiber path 525.

In one implementation, hub 510 determines whether to switch from primary fiber path 520 to the backup fiber path 525 based on all of the signal quality feedback provided for the remote nodes 530-1 to 530-N. For example, in one implementation, if a downlink BER or optical power level reported by any one of the remote nodes 530-1 to 530-N indicates degrading optical signal quality, but downlink BERs or optical power levels reported by the other remote nodes do not, the hub 510 concludes that the degrading optical signal quality is due to a local problem with the one remote node, rather than a degradation of the primary fiber path 520. However, when all of the remote nodes 530-1 to 530-N report degrading downlink optical signal quality, then hub 510 concludes that the degrading optical signal quality is due to a degradation of the primary fiber path 520 and switches to the backup fiber path 525. Similarly, if hub 510 detects the loss of an uplink optical signal from one of the remote nodes 530-1 to 530-N, but continues to detect uplink optical signals from the other remote nodes, the hub 510 concludes that the degrading optical signal quality is due to a local problem with that remote node, rather than a degradation of the primary fiber path 520. However, when hub 510 detects a loss of uplink optical signals from all of the remote nodes 530-1 to 530-N, then hub 510 concludes that the degrading optical signal quality is due to a degradation of the primary fiber path 520 and switches to the backup fiber path 525.

It would be appreciated by one skilled in the art upon reading this specification that the embodiments of the present invention are not limited to the transport of analog RF signals but includes the transport of analog RF signals and digital RF signals, or any combination thereof in either the uplink or downlink directions. For example, in one implementation of a network such as those describe above, one or more of a hub's interface functionality communicate wirelessly to one or more base stations via digital RF signals while one or more of the remote nodes communicate wirelessly via analog RF signals. Such an implementation may include, but is not limited to a Universal Mobile Telecommunications System/Wideband Code Division Multiple Access (UMTS/WCDMA) network. 

What is claimed is:
 1. A system comprising: a hub; at least one remote node that is located remotely from the hub; wherein the hub communicates with the at least one remote node via either one of a primary fiber path and a backup fiber path, the primary fiber path comprising an uplink fiber and a downlink fiber and the backup fiber path comprising an uplink fiber and a downlink fiber; wherein the at least one remote node is coupled to the downlink fiber of the primary fiber path and the downlink fiber of the backup fiber path via an optical combiner; wherein the at least one remote node is further coupled to the uplink fiber of the primary fiber path and the uplink fiber of the backup fiber path via an optical splitter; wherein the at least one remote node further comprises a manager monitoring a signal quality of a downlink optical signal, wherein the manager transmits an uplink optical signal to the hub based on the signal quality; and wherein the hub switches from communicating with the at least one remote node on the primary fiber path to communicating with the at least one remote node on the backup fiber path based on the uplink optical signal from the manager.
 2. The system of claim 1, wherein when the hub detects a loss of the uplink optical signal from the uplink fiber of the primary fiber path, the hub switches from communicating with the at least one remote node on the primary fiber path to communicating with the at least one remote node on the backup fiber path.
 3. The system of claim 1, wherein when the hub detects a loss of the uplink optical signal from the uplink fiber of the backup fiber path, the hub switches from communicating with the at least one remote node on the backup fiber path to communicating with the at least one remote node on the primary fiber path.
 4. The system of claim 1, wherein the uplink optical signal comprises multiple wavelength optical signals.
 5. The system of claim 1, wherein one or both of the optical combiner and the optical splitter are located external to the at least one remote node.
 6. The system of claim 1, wherein one or both of the primary fiber path and the backup fiber path comprises a plurality of uplink fibers.
 7. The system of claim 1, wherein one or both of the primary fiber path and the backup fiber path comprises a plurality of downlink fibers.
 8. The system of claim 1, wherein the at least one remote node is adapted to determine a signal quality of a downlink optical signal received from the downlink fiber of one or the primary fiber path and the backup fiber path; and wherein the at least one remote node is further adapted to communicate the signal quality of the downlink optical signal to the hub.
 9. The system of claim 8, wherein the at least one remote node is further adapted to determine one or both of the bit error rate of the downlink optical signal and the optical power level of the downlink optical signal.
 10. The system of claim 8, wherein the at least one remote node is further adapted to alert the hub of an inadequate signal quality of the downlink optical signal by disabling the uplink optical signal.
 11. The system of claim 8, wherein when the at least one remote node communicates to the hub that the signal quality of the downlink optical signal from the primary fiber path is inadequate, the hub switches from communicating with the at least one remote node on the primary fiber path to communicating with the at least one remote node on the backup fiber path.
 12. The system of claim 8, wherein when the at least one remote node communicates to the hub that the signal quality of the downlink optical signal from the backup fiber path is inadequate, the hub switches from communicating with the at least one remote node on the backup fiber path to communicating with the at least one remote node on the primary fiber path.
 13. The system of claim 8, wherein the downlink optical signal comprises multiple wavelength optical signals.
 14. The system of claim 1, wherein the hub is adapted to determine a signal quality of an uplink optical signal received from the uplink fiber of one of the primary fiber path and the backup fiber path; wherein when the uplink optical signal received from the uplink fiber of the primary fiber is inadequate, the hub is adapted to switch from communicating with the at least one remote node on the primary fiber path to communicating with the at least one remote node on the backup fiber path; wherein when the uplink optical signal received from the uplink fiber of the backup fiber is inadequate the hub switches from communicating with the at least one remote node on the backup fiber path to communicating with the at least one remote node on the primary fiber path.
 15. The system of claim 14, wherein the at least one remote node is further adapted to determine the bit error rate of the downlink optical signal.
 16. A method comprising: transmitting a downlink optical signal to a downlink fiber of one of a first fiber path and a second fiber path; combining any downlink optical signal received on the downlink fiber of the first fiber path and any downlink optical signal received on the downlink fiber of the second fiber path in order to output the downlink optical signal; splitting an uplink optical signal for transmission on an uplink fiber of the first fiber path and an uplink fiber of the second fiber path; determining a signal quality of the downlink optical signal as received at a first unit; determining a signal quality of the uplink optical signal as received at a second unit; switching at the second unit between a normal state and backup state based on one or both of the signal quality of the uplink optical signal and the signal quality of the downlink optical signal; wherein the first unit communicates the signal quality of the downlink optical signal to the second unit.
 17. The method of claim 16, wherein determining a signal quality of the downlink optical signal further comprises transmitting a signal quality indication via the uplink optical signal when the signal quality of the downlink optical signal is inadequate.
 18. The method of claim 16, wherein determining a signal quality of the downlink optical signal further comprises disabling transmission of the uplink optical signal when the signal quality of the downlink optical signal is inadequate.
 19. The method of claim 16, wherein determining a signal quality of the downlink optical signal further comprises calculating one or both of the bit error rate of the downlink optical signal and the optical power level of the downlink optical signal.
 20. The method of claim 16, wherein determining a signal quality of the uplink optical signal further comprises calculating one or both of the bit error rate of the uplink optical signal and the optical power level of the uplink optical signal.
 21. The method of claim 16, wherein one or both of the downlink optical signal and the uplink optical signal comprise multiple wavelength optical signals.
 22. A distributed antenna system, the system comprising: means for combining an optical signal received on a downlink fiber of a first fiber path with an optical signal received on a downlink fiber of a second fiber path in order to output a downlink optical signal; means for splitting an uplink optical signal for transmission on an uplink fiber of the first fiber path and an uplink fiber of the second fiber path; means for determining an attribute of the downlink optical signal indicative of the signal quality of the downlink optical signal as received at a first unit; means for determining an attribute of the uplink optical signal indicative of the signal quality of the uplink optical signal as received at a second unit; and means for switching between a normal alignment state and a backup alignment state based on one or both of the attribute of the downlink optical signal and the attribute of the uplink optical signal wherein the first unit communicates the attribute of the downlink optical signal to the second unit.
 23. The system of claim 22, further comprising: means for communicating the signal quality of the downlink optical signal via the uplink optical signal.
 24. The system of claim 22, wherein one or both of the downlink optical signal and the uplink optical signal comprise multiple wavelength optical signals.
 25. The system of claim 22, wherein one or both of the means for combining and the means for splitting are located externally from the means for determining an attribute of the downlink optical signal, the means for determining an attribute of the uplink optical signal and the means for switching. 